1. Field of the Invention
The present invention relates to an explosive detection system. Specifically, the present invention relates to a detection system using thermal neutrons in combination with a detector formed from an inorganic scintillator such as a scintillator made of sodium iodide to provide for a more efficient detection of explosives. A specific detection system may be provided using a ring of detectors to detect the presence of explosive within a particular plane of an object under inspection and with a continuous movement of the object providing for a three dimensional profile of the explosive.
2. Description of the Prior Art
A great need exists for the scanning of luggage, baggage and other parcels for the detection of any explosive material contained or concealed within their confines. For example, a large number close to two million (2,000,000) pieces of luggage are checked and/or carried onto aircraft daily by close to seven hundred and fifty thousand (750,000) passengers within six hundred (600) airports extending across the country. Many more packages move through the mails or are shipped to sensitive buildings. There is a possibility, albeit small, that any one piece of luggage or parcel may contain explosive material. It is, therefore, desirable to protect the public by providing detection systems to scan the luggage and parcels to detect the presence of any explosive material.
It thereby follows that any system of checking luggage or parcels must have a very high probability of detection in order to be effective. Because of the large number of parcels processed, a high throughput is necessary for practicality. In addition, because of the large number of scanned items, any detection system is bound to occasionally give a false alarm. The probability of these false alarms must be minimized in order to provide for an effective explosive detection system. This is true since when an alarm occurs it is not known, at that time, whether it is true or false. This means that each time an alarm occurs, a passenger or parcel must be detained for further investigation. If false alarms are significantly high the nuisance level and the delays could be unacceptable to the public. It is, therefore, important that any explosive detection system must have a very high probability of detection and a high throughput rate, and yet, at the same time, have a very low probability of false alarms. These conflicting criteria have hampered efforts in the past to build a reliable and usable system
In general, prior art systems have not met the desired characteristics of having a high probability of detection with a low probability of false alarms at acceptable throughput rates. As an example, one such prior art system is shown in U.S. Pat. No. 3,832,545. This patent provides for a system for the detection of nitrogen, which is generally present in the explosive materials to be detected. The object under observation is positioned within a cavity structure and the object is bombarded by thermal neutrons. The thermal neutrons interact with any nitrogen contained in the object to induce the emission of gamma rays at an energy level characteristic of the nitrogen element.
The emitted gamma rays are then detected by two parallel planar arrays of gamma ray detectors. U.S. Pat. No. 3,832,545 specifically provides for the use of liquid or plastic type organic scintillator detectors having an end surface for viewing a portion of the article being inspected and with the length of the organic scintillator being substantially greater than the effective diameter of the end surface. As described in this prior art patent, the array of organic scintillators provides for a crude two dimensional profile of the nitrogen content within the object being inspected. The two dimensional concentration profile of the nitrogen is then used to provide for a detection of an explosive. This type of prior art system has a number of deficiencies, including both a low gamma ray intensity and spatial resolution of the detection of the concentration of the nitrogen contained in the object under inspection, and the insensitivity of the system to detect explosive devices which are deliberately positioned within the object under inspection so as to defy detection. Because of the use of liquid or plastic type scintillators, long times are required to make a decision about any object. The system described in the prior art patent is also slow and cumbersome in operation which is a further limitation to its usefulness.
Other types of prior art explosive detection systems depend upon the prior seeding of explosive materials with a tracer material, such as a radioactive tracer. Although this type of system could be very useful if all explosive material were manufactured with such tracer material, because of the large amount of explosive material which has already been manufactured and because of the difficulty of controlling the manufacture of all explosive material so as to contain such tracer material, this type of system is not practical. A usable system must be able to detect the presence of explosive material of a conventional type and of an unconventional type, whether disposed within an object either in its original manufactured form, or if deployed within the object so as to attempt to confuse or evade the detection system. The prior art systems have not met these various criteria and cannot produce the desired high probability of detection with the relatively low production of false alarms.
An acceptable response to the explosive threat to aviation, mails, or shipping requires detection techniques that are highly sensitive, specific, rapid and non-intrusive. The efficient detection of nitrogen, at this point, offers the best overall solution. It is, therefore, important that this detection of nitrogen be provided to give the maximum information of the physical parameters of the explosive, such as density and spatial distribution The use of nuclear based techniques which subject the luggage or parcels to thermal neutrons can be the basis of a system to produce the desired results, but this system cannot be based on the prior art techniques. It is important that the intensity, energy and spatial distribution of the detected radiations from the object under observation must be provided in such a way so as to help to determine the presence or absence of explosives and this has not yet been accomplished.
In addition to high detection sensitivity and low false alarm the detection of the explosive should be independent of the specific configuration and must be non-intrusive in order to protect privacy. The detection equipment, of course, must be non-hazardous to the contents of the checked items and to the operating personnel and environment. Other more general criteria are that the system must be reliable, easily maintained and operable by relatively unskilled personnel and that the cost must be low enough so as to be non-burdensome to airlines and airports. Finally, it is desirable, when all other requirements achieved that the size of the system be relatively small so that the system may be useful in a wide variety of environments.
In addition to the nuclear based systems described above, non-nuclear systems have also been investigated. These systems have achieved relatively high efficiencies of detection, but generally have relatively high false alarm rates and have long screening times. These type of non-nuclear systems, therefore, by themselves cannot achieve the desired results. It is possible to combine a non-nuclear system with a nuclear system, but the present invention is directed to specific improvements in the nuclear based type of system.
In order to develop a proper explosive detection system, an understanding is required of the properties of the various explosives relevant to the specific techniques to be used. Although there are a large number of explosive types, a general classification into six major groups with minor variations, has been proposed. The proposed classification scheme includes the following types of explosives: (1) nitroglycerine based dynamites, (2) ammonium nitrate based dynamites, (3) military explosives, (4) homemade explosives, (5) low order powders, and (6) special purpose explosives.
Nitroglycerine based dynamites are the most common form of explosives. The basic composition includes equal amounts of nitroglycerine and ethylene glycol dinitrate, plus a desensitizing absorber in the form of cellulose in either sodium or ammonium nitrate.
The ammonium nitrate based dynamites have been replacing nitroglycerine based dynamites in popularity. These types of dynamites are commonly referred to as slurries or water gels. The two general types of ammonium based dynamites are the cap-sensitive and the cap-insensitive types. The former consists of aluminum, ammonium nitrate, ethylene glycol and water while the latter contains wax or fuel oil and water.
Military explosives are formed of Composition-4 (C-4), TNT and picric acid. C-4 is composed of cyclotrimethylene trinitramine (RDX) and a plasticizer.
Homemade explosives are diverse and are limited only by the creativity of the perpetrator. Ammonium nitrate (fertilizer) and fuel oil are the most common and available constituents.
Low order powders (black and smokeless) have typically been assembled in pipe bomb configurations and have been used extensively in that form. Black powder contains potassium nitrate, carbon and sulfur. Smokeless powder is primarily pure nitrocellulose or a mixture of nitrocellulose and nitroglyerine.
Special purpose explosives include detonating cords, blasting caps and primers. The explosive entities in the special purpose explosives are PETN, lead azide, lead styphanate, mercury fulminate and blasting gel.
In general, all of these explosives contain a relatively high amount of nitrogen ranging from nine to thirty five percent by weight. The nominal density of these explosives is typically 1.6 gm/cm.sup.3 and with ranges between 1.25 to 2 gm/cm.sup.3 or more. These physical properties demonstrate that the most unique signature of explosives is the high concentration and density of the nitrogen content. There are other physical factors that identify explosives, but these other factors do not form part of the present invention. However, one factor which is important is that most explosives have a minimum progagation thickness or diameter in order to be effective. The minimum propagation thickness entails a sizable contiguous body of explosives in the other two dimensions. This information is useful to the detection of explosives without making a specific assumption of the actual shape of the explosive.
In can be seen, therefore, that a nuclear detection technique can provide for the detection of the nitrogen content to reliably indicate the presence of a large nitrogen content. However, the frequent occurrence of nitrogen in non-explosive materials limits the level of detection sensitivity and merely detecting the presence or absence of nitrogen alone is not sufficient. Therefore, additional information is required beyond simply sensing the presence of the nitrogen. The present invention provides for this additional information using specific structures and a specific detection configuration to provide for a greater reliability in the detection of explosives.